Preheat your oven to 350°F (175°C).
In a 9x13 baking dish, melt the butter and mix in the brown sugar until well combined, then spread this mixture evenly across the bottom of the dish.
Layer the slices of bread on top of the sugar-butter mixture.
In a separate bowl, whisk together the eggs, whole milk, heavy whipping cream, and vanilla until everything is well blended.
Pour half of the egg mixture over the first layer of bread, then sprinkle a bit of cinnamon on top.
Add another layer of bread, then pour the remaining egg mixture over it, pressing down gently to ensure the bread soaks up the liquid.
Sprinkle more cinnamon or cinnamon sugar on top, cover the dish, and refrigerate it to soak overnight or at least until you're ready to bake.
When you're ready to bake, cover the dish with aluminum foil and bake for 30 minutes.
After 30 minutes, remove the foil and continue baking uncovered for an additional 15 minutes, checking for doneness; if it still jiggles, bake uncovered for another 10–15 minutes until it is set.
Let the casserole cool slightly before cutting it into portions, and serve with maple syrup and optional powdered sugar.
